This invention relates generally to semiconductor packaging, and in particular it relates to the packaging of injection laser arrays.
Injection lasers, particularly those of the double-heterojunction gallium arsenide (DH GaAs) type, are of very small size and yet are able to generate powerful and highly concentrated light beams. For these reasons it has been proposed to utilize arrays of such lasers closely spaced together in situations where multiple-beam scanning is desired. One proposed application of multiple-beam scanning is thermal transfer printing, wherein a one-dimensional array of closely spaced lasers generates a plurality of beams which will be caused to scan in unison along parallel paths on a thermal transfer ribbon positioned against a paper, each of the beams being appropriately modulated by on-off switching actions for causing selected elements of characters to be printed in a particular row of each character matrix, the result being that an entire line of characters is printed during each scan of the multiple beam group across the paper.
In an application of this kind it is preferable that the laser array be designed to operate in air at room temperature without overheating so that the apparatus will not be encumbered with cooling equipment of the kind needed to maintain very low ambient temperature. Operation of injection lasers in such close proximity at room temperature gives rise to a heat dissipation problem which is made more severe by the fact that in this kind of array, where each laser must be capable of switching on and off individually as needed during each scan of the beam, it is necessary to make isolated electrical switching connections to the individual lasers on their "hot" sides, where the controllable active regions of the lasers are located. This makes it impossible to mount the lasers so that their hot sides are in direct contact with a common heat sink such as a copper block, because the common interconnection provided by such a heat sink would obliterate the individual switching connections that are required. The other side of the laser array, where a common electrical return connection can be made, cannot be used as a heat sink inasmuch as the relatively thick substrate between the hot layers and the common return has a poor thermal conductivity. Thus, the only side of the injection laser array which is permitted to have direct contact with an electrically conductive member common to all of the lasers is the side where the thermal conductivity is poor, whereas the side of the array where the thermal conductivity is good cannot be mounted directly upon a common heat sink because of the requirement that isolated switching connections be made to the respective lasers on that side of the array.
Prior attempts to resolve this dilemma have not met with notable success. Generally it has been the practice in designing this type of array to accept relatively inefficient heat dissipation as the price which must be paid to satisfy the requirement for electrical isolation among the individual laser switching circuits. This practice has tended to reduce the value of laser technology for applications of the kind mentioned above.